Conventionally, heat insulating structures for heat insulating an internal combustion engine including a cylinder block and a cylinder head coupled to the top of the cylinder block are known in the art. Patent Document 1 discloses a heat insulating structure of an internal combustion engine configured as an engine encapsulation structure including an engine compartment encapsulation member surrounding the upper portion of a power train configured as an assembly of an engine (internal combustion engine) and a transmission in an engine compartment, and an underbody encapsulation member surrounding the lower portion of the power train. The engine compartment encapsulation member and the underbody encapsulation member are provided such that the engine compartment encapsulation member and the underbody encapsulation member are vertically assembled together in a space between the power train and a vehicle body to surround the power train, and that air, for cooling the power train, flowing from an inlet of a front surface in the state where the engine compartment encapsulation member and the underbody encapsulation member are vertically assembled together is discharged from an outlet of a rear surface where the underbody encapsulation member is opened.